Baldwin: Weed species shift vs. herbicide resistance

I have been attempting in recent columns to bring in what history can teach us about the herbicide resistance issues we are facing. One colleague who has attempted to keep me straight through the years recently e-mailed a comment along the lines that “I do not see how weed species shifts are any different from herbicide resistance, although you seem to think so.”

In the big picture the end result from a species shift or a weed becoming resistant are essentially the same. With a species shift we used the same herbicide or program repeatedly until a different weed species took the place of those we were effectively controlling. In the recent articles I used the examples of prickly sida, cocklebur and morningglories replacing the annual grasses and pigweeds we were effectively controlling with Treflan and similar herbicides of the day. There are plenty of other examples I could have chosen.

There is a principle in plant ecology that says plants are going to occupy open space. When we effectively removed the grasses and pigweeds, it left open space and new weeds moved in. Therefore, while the herbicide or program did not become ineffective because the weeds that were previously being controlled became resistant, the end result was those herbicides or programs no longer provided acceptable weed control.

When herbicide resistance is being discussed, it is in the context of a weed species that was previously effectively controlled but is no longer controlled. This can happen as the result of a resistant plant being present in a population and the herbicide controls the susceptible plants but leaves the resistant plant. As a result, only resistant plants are reproducing and before long only resistant plants are left.

The same thing can happen if a mutation occurs in the weed population and the herbicide is rendered ineffective. There can also be what some have termed a creeping resistance where over time plants just build up a tolerance that eventually leads to resistance. While in one sense the development of resistance is different from a species shift, the end result for the farmer is the herbicide or program no longer works.

Another reader asked, “What is the difference in a weed becoming resistant to a conventional herbicide and one becoming resistant to a herbicide like glyphosate where a trait is involved?” There really is not any difference. That is why I say “baloney” to those who say glyphosate has created a super weed in Palmer pigweed. Palmer pigweed was already a super weed, but is now tolerant or resistant to one more herbicide mode of action. The reason I have likely sounded like glyphosate resistance is different has more to do with the timing and magnitude of the problem rather than the principle.

There are two reasons I have attempted to present the glyphosate resistance in a different light compared to species shifting and resistance to conventional herbicides.

The first reason is we are living in a different time now. As I have written in several articles, there was a time when the development pipeline was full of new herbicide candidates. As species shifts occurred or as resistance developed to most of the conventional herbicides, we had new herbicides coming along to solve the problem. That pipeline is now empty. To a great extent those easy answers are no longer at our finger tips.

The other reason I have presented glyphosate resistance in a different light is there has simply never been a weed control technology that brought so much to the table. While some colleagues have argued differently, Roundup Ready is miracle technology. Therefore we have never had this much to lose with a species shift or with the development of resistance to a single herbicide.

Those two things put glyphosate resistance in a world of it’s own in this weed scientist’s opinion!